(1) Field of the Invention
The present invention relates to an oxygen sensor element and its manufacturing process. More specifically, the invention relates to the manufacture of an oxygen sensor and an oxygen sensor element which is used as a unit for detection of oxygen concentration in the oxygen sensor to determine the concentration of oxygen in exhaust gas in an emission control system for disposing of the three harmful components of auto engine emissions, i.e., unburnt hydrocarbons, carbon monoxide and nitrogen oxides, simultaneously through reaction with a catalyst ("three-way system").
(2) Description of the Prior Art
Generally an oxygen sensor is equipped with an oxygen sensor element which is an oxygen concentration cell consisting of a solid electrolyte of a specific ceramic material with oxygen ion conductivity, in which the oxygen concentration on the measured gas side is determined by measuring the electromotive force produced through a difference in the oxygen partial pressure between the measured gas side and the reference gas side.
In the conventional process of manufacturing the oxygen sensor element, a solid electrolyte vessel with one end open is prepared; on the inside and outside surfaces of this vessel a porous metal electrode layer is formed; a metal or a mixture of a metal and its oxide as the reference oxygen partial pressure-generating substance is charged into the vessel and sintered; a lead wire is put through the opening of the vessel and into contact with the metal electrode layer on the inside surface of the solid electrolyte vessel; and then the opening of the solid electrolyte vessel is sealed, with the lead wire running through the seal.
In such a process, however, molding or otherwise forming the solid electrolyte in the form of a vessel is complicated and hinders mass production of the oxygen sensor element. In addition, the formation of the required metal electrode layer on the inside surface of the solid electrolyte vessel is difficult to control, and hence uniform quality oxygen sensor elements are difficult to make. Moreover, because the solid electrolyte vessel is formed, and then filled with a metal or a mixture of a metal and its oxide which are then sintered, heat and stress are developed in the solid electrolyte vessel and the metal electrode layer resulting in changes of the electrical, mechanical and thermal properties, and causing further variations in quality of the oxygen sensor elements. Furthermore, the liability to such troubles leads to a great bother in the control of the manufacturing steps.
Meanwhile, because of the necessity to keep the reference oxygen partial pressure of the sintered metal (or a sintered mixture of a metal and its oxide) at a constant level, the opening of the solid electrolyte vessel must be sealed and accordingly, a special seal is needed to assure perfect airtightness. This is another problem area which complicates the manufacture of the oxygen sensor element and renders it impossible to make the oxygen sensor element itself compact.